The transmission electron microscope (TEM) has been used to examine solid materials since its invention in the 1940s. But liquid materials are much harder to examine. This is because the interior of the TEM is maintained at a high vacuum and most liquids, especially water-based solutions, would quickly evaporate into the vacuum before observations could be made.
“Liquid cells” are used in the prior art with TEM to examine electrochemical and other reactions in liquids. One such electrochemical cell (liquid cell), shown in FIG. 1, was designed, developed, and built at IBM's T. J. Watson Research Center and has proven successful in observing electrochemical reactions that take place in water-based solutions. Prior art cell design and some of the results obtained using such prior art are described in the following references, which are herein incorporated by reference in their entirety:    (1) M. J. Williamson, R. M. Tromp, P. M. Vereecken, R. Hull and F. M. Ross, Dynamic electron microscopy in liquid environments, Nature Materials 2, 532-536 (2003).    (2) A. Radisic, P. M. Vereecken, J. B. Hannon, P. C. Searson and F. M. Ross, Quantifying electrochemical nucleation and growth mechanisms from real-time kinetic data. Nano Letters 6, 238-242 (2006).    (3) E. P. Butler and K. F. Hale, Dynamic Experiments in the Electron Microscope. Elsevier (1981).    (4) Hummingbird Scientific, WA; http://www.hummingbirdscientific.com/PdfFiles/LiquidCellHolder.pdf
Refer to the prior art electrochemical cell 100 shown in FIG. 1 in an isometric view and FIG. 2 shown in cross section 200.
The basic electrochemical cell design 100 consists of two silicon wafers, a bottom substrate 105 and a top substrate 115. A thin bottom insulator 108 (preferably made from a layer of silicon nitride, SiN, or other material such as silicon carbide) covers the bottom substrate chamber side 106 and a top insulator 111 covers the top substrate chamber side 116. Small areas of the two wafers are etched to remove the substrate but leave the insulator 108 and 111, thereby forming the top window 135 and bottom window 140; the bottom 108 and top 111 insulators form a cover over the bottom 140 and top 135 windows. The bottom substrate 105 has a backside 118 and the top substrate 115 has a backside 117. Both backsides (117 and 118) are opposite the respective chamber sides (106, 116). A spacer 107 is patterned on the bottom insulator 108. The spacer 107 is preferably in the shape of a rectangular outline surrounding the window, and is made from a material such as silicon dioxide. The wafers are then glued chamber side to chamber side so that the top window 135 and bottom window 140 are aligned, 197, and the spacer layer keeps them a fixed distance apart. This alignment allows the electron beam path 198 to pass through the top 135 and bottom 140 windows so that an image can be formed from transmitted electrons through the path 198 onto a detector/camera (not shown). The liquid/electrolyte under study 190 is introduced between the two substrates (105, 115) to form a thin layer, using a syringe to inject it (see below). If the two windows (135, 140) and the liquid layer, i.e., layer of electrolyte between the windows (135, 140) are thin enough for electrons from the TEM to pass through them, then the liquid and windows can be examined successfully in the TEM. The idea of encapsulating a liquid/electrolyte between SiN windows is well known.
In order to allow the liquid to be introduced easily, additional components of the cell were added to this original design. Thus, the complete design also includes features such as two tanks 125 to contain reservoirs 120 of the liquid 190, and two lids 130 to seal up the tanks. These components are attached to each other using glue. Apertures are placed in the top substrate 115 so that the electrolyte 190 in the tanks 125 and the space between the top 135 and bottom 140 windows is in fluid communication.
In addition, to allow electrochemical reactions to be observed two or three electrodes are required (160, 170, 180) which must be inserted into or to be in contact with the liquid. Each electrode must have part in contact with the liquid and part outside to allow electrochemical measurements to be made by connecting an external current or voltage source. The cell design shown in FIG. 1 gives one original method by which these electrodes were introduced. One electrode (the working electrode internal contact 155) is patterned so that part of it overlaps the bottom window 140. It is connected electrically outside the cell by patterning it over a via (hole) 109 that had been etched through the insulator 108 in a previous processing step, and by patterning a contact pad, the working electrode external contact 145 over a similar via near the edge of the cell. Electrical contact thus took place from the internal contact 155, through the Si wafer 150, out to the external contact 145. This allowed the external electrical contact 145 in the prior art to be outside the spacer 107. The other two electrodes, the reference electrode 170 and the counter electrode 180, were made of thin wires inserted manually into the cell through the topmost glue layer 181.
The cells built using this design were successful, but had yield problems that were associated with the design of the electrodes. The two electrodes made of wire 170 and 180 had to be placed manually and the caused leaks in the glue layer 181 between the lid 130 and the tanks 125 allowing the liquid to escape before the cell could be used. Even more significantly, the fabrication of the vias 109 and the patterning of the working electrode (both the internal 155 and external 145 contacts) were difficult. This was due to problems associated with the electrical connection between the electrode metal and the silicon wafer. In the prior art, the electrical current passed from an electrical source to the external electrode contact 145 and then through the bottom substrate 150 before flowing through the internal contact 155 and eventually to the electrolyte 190. The exposed substrate surface does not make a reliable contact with metal contacts such as 145 and 155 probably due to oxidation of the silicon, causing a high resistance between the contacts 145 and/or 155 and the bottom substrate 150 that inhibited current flow to the electrolyte 190. The poor and unreliable electrical contact between the two parts of the working electrode resulted in a low yield of working cell substrates, and the performance of each electrode had to be tested by making individual measurements which increased the process time drastically.